1. Field of the Invention
The present invention relates to a discharge generating apparatus and a discharge generating method and, more particularly, to a discharge generating apparatus and a discharge generating method capable of generating a plasma applicable to decomposition of an organic compound, surface treatment, etc. using the energy of a plasma, by generating a plasma from a gas in a glow discharge under ordinary pressure.
2. Related Background Art
A number of methods have been developed heretofore generally as methods of generating a discharge under ordinary pressure. The methods can be roughly classified under a silent discharge, a creeping discharge, a pulse corona discharge, and so on. Among these, the silent discharge is known as ozone generators, in which the electrode-to-electrode distance is very short, about several mm, and a lot of reactors need to be prepared in order to attain an stable discharge with a large amount of a gas. The creeping discharge is a discharge generated along a ceramic surface with application of alternating electric field between electrodes set on the surface and inside an inorganic material such as ceramics or the like, which is an extremely local discharge state. As for the pulse corona discharge, the rise time of the electrodes is quick, 10 to 50 ns, streamer corona is generated in the electrode space with application of high voltage of extremely short pulses having the half width of 50 to 500 ns, and the power supply, electrodes, etc. are expensive. There are also proposals of methods of establishing an stable glow discharge under the atmospheric pressure with some measures on the electrodes, but they still have disadvantages including the need for an expensive gas such as helium or argon, extreme narrowness of a stable discharge part, and so on.
Therefore, there exist discharge generating methods making use of inorganic dielectrics, as methods of stably generating a discharge under ordinary pressure while overcoming these disadvantages. These are methods of generating the plasma by applying ac voltage or pulse voltage between electrodes with the inorganic dielectric in between. More specifically, the voltage from a high voltage ac power supply (ac voltage supply) or a pulse power supply (pulse voltage supply) is placed between opposed electrodes to establish a potential difference between the electrodes, whereupon polarization occurs inside the inorganic dielectric filling the space between the electrodes to generate a pulsed microdischarge of the ns order. This microdischarge changes the gas into a plasma.
According to the above discharge generating methods making use of the inorganic dielectrics, however, since the voltage necessary for generation of a glow discharge is dependent upon the distance between the electrodes, the voltage necessary for generation of arbitrary electric current increases with increase in the distance between the electrodes. For that reason, there is a problem that scale increase of the discharge generating apparatus using these methods is restricted by setting of the distance between the electrodes in the prior art. This problem was hindrance to scale increase in terms of design of apparatus. Describing this point in further detail, in the plasma formation by these methods, for example, in a process wherein a gas under flow is continuously and efficiently converted into a plasma, since increase of flow rates can induce influence of pressure loss due to the dielectric filling, the flow passage needs to be set large, and thus the distance between the electrodes has to be increased. Consequently, it also requires increase of input voltage in order to maintain decomposition efficiency and this increase of voltage poses the problems of increase in the size of power supply, or restrictions on the discharge conditions. The increase in the distance between the electrodes made it unfeasible to convert a large amount of a gas into a plasma at high efficiency by the prior art methods, because of limitations to the safety, construction of apparatus, and so on.
The present invention solved the above problems and an object of the invention is to provide a discharge generating apparatus and a discharge generating method that can generate a large amount of a plasma, that can generate the plasma under ordinary pressure, and that can increase the processing efficiency of the plasma processing, gas decomposition, and so on.
Specifically, an object of the present invention is to provide a discharge generating apparatus comprising high-potential and low-potential electrodes, at least either one of the high-potential electrode and the low-potential electrode comprising a plurality of electrodes, and having such a cinfiguration that a space between the electrodes is filled with an inorganic dielectric having a structure permitting flow of a gas, wherein a discharge is generated between the electrodes to change the gas existing between the electrodes, into a plasma.
Another object of the invention is to provide a discharge generating method comprising the steps of placing high-potential and low-potential electrodes, at least either one of the high-potential electrode and the low-potential electrode comprising a plurality of electrodes, and filling a space between the electrodes with an inorganic dielectric having a structure permitting flow of a gas, wherein a discharge is generated between the electrodes to change the gas existing between the electrodes, into a plasma.
In the above discharge generating apparatus or discharge generating method, it is preferable that the gas existing between the electrodes be changed into a plasma under ordinary pressure or that the discharge be a glow discharge.
It is also preferable that the high-potential and low-potential electrodes are arranged in alternate fashion or that the inorganic dielectric be filled between one high-potential electrode and at least one low-potential electrode.
It is also preferable that the high-potential electrode be connected to an ac voltage supply or to a pulse voltage supply or that the low-potential electrode be grounded.
It is also preferable that the high-potential electrode be connected through an electric conductor to the ac voltage supply or to the pulse voltage supply or that the low-potential electrode be grounded through another electric conductor.
It is also preferable that one of the high-potential electrode and the low-potential electrode be two or more internal electrodes and that the other be a cylindrical electrode surrounding the internal electrodes.
It is also preferable that the shape of the internal electrodes be a rodlike shape or a platelike shape.
It is also preferable that the internal electrodes be connected to the ac voltage supply or to the pulse voltage supply and that the cylindrical electrode be grounded.
It is also preferable that the inorganic dielectric be filled in the presence of a gap permitting flow of the gas between the internal electrodes and the cylindrical electrode.
It is also preferable that the discharge be utilized for decomposition of an organic compound existing in the gas.
It is also preferable that the inorganic dielectric have a structure with gaps permitting flow of the gas and generating the discharge in the gaps of the inorganic dielectric.
It is also preferable that the inorganic dielectric be a ferroelectric material and it is particularly preferable that the inorganic dielectric be comprised of at least one selected from barium titanate and strontium titanate. The inorganic dielectric is preferably a granular material.